1. Field of the Invention
This invention relates to a direct current resistance welding machine and a method of controlling the direct current resistance welding machine.
2. Description of the Background Art
Varying the frequency of a primary current in the welding transformer according to variations in a load on the secondary side, in an inverter-type direct current resistance welder or welding machine, has been disclosed in Japanese Patent Application Laid-Open Publication No. 63-273575. In such an inverter-type direct current resistance welding machine, the welding transformer is controlled within a range of the frequency at which the iron core of the welding transformer is magnetically unsaturated, by supplying a primary current of a high frequency when the value for setting a welding current is small or, supplying a primary current of a low frequency when the welding current setting value is large.
When the conventional control such as slow-up control for successively increasing the welding current at the beginning of energization, waveform control effected for the duration of energization, etc. is effected, the frequency of the primary current varies each time according to the state of the load on the secondary side. The frequency of the primary current of the welding transformer thus cannot be accurately set up.
Therefore, an operator usually made combinations of test pieces and their corresponding welding guns and measured welding conditions including the frequency of the primary current. Further, the operator inputted the measured welding conditions to a control unit in advance. Then, the operator specified the welding conditions which had been inputted in advance based on the objects to be welded and the welding gun to be used before welding. Thereafter, the welding was carried out under the specified welding conditions. Complex welding conditions such as slow-up control, waveform control, etc., required much time to determine.
According to the inverter type direct current resistance welding machine, it has been known that the iron core of the welding transformer can be reduced in sectional area by increasing the frequency of the primary current, thereby making it possible to reduce the welding transformer size and weight.
In the method of reducing the welding transformer in size by increasing the frequency of the primary current, however, an output voltage V.sub.0 is greatly reduced at a predetermined frequency, e.g., 1 kHz or higher according to time intervals required for the primary current of the welding transformer to rise and fall. Thus, even if the welding transformer is reduced in size by simply increasing the frequency, the desired output cannot be obtained.